This invention relates to an ink jet recording apparatus for recording an image, text, etc., on recording medium with a recording head and a driving method of an ink jet recording head and in particular to an ink jet recording apparatus adapted to jet an extremely small amount of ink drop capable of forming a microdot.
An ink jet printer is already well known as a representative ink jet recording apparatus. With the printer, the dot diameter on recording paper, namely, image quality (resolution) is determined by the amount of ink drop jetted from a recording head. Thus, to control the jetting amount of ink is important.
If the jetting amount of ink drop is controlled based on the diameter of a nozzle orifice, if the diameter is small, the resolution can be improved, but the recording speed becomes low; if the diameter is large, the recording speed can be increased, but a coarse image low in resolution is formed.
To meet such opposed requirements, there is an idea which can jet different volume of ink through the same nozzle orifice of a printer.
When an extremely small dot (microdot) is recorded by the printer, a pressure generating element such as a piezoelectric actuator is driven by a drive signal containing drive pulses appropriately for jetting different amounts of ink drops.
An ink drop is jetted using change in ink pressure in a pressure generating chamber accompanying expansion and contraction thereof.
By the way, more improvement in image quality has been demanded for recent ink jet recording apparatus. With the drive pulse in the related art mentioned above, a small amount of ink drop can be jetted. However, jetting a smaller amount of ink drop is demanded from the viewpoint of improvement in recording quality.
It is therefore an object of the invention to provide a driving method of an ink jet recording head capable of jetting an extremely small amount of ink drop, and an ink jet recording apparatus incorporating the same.
In order to achieve the above object, there is provided an ink jet recording apparatus comprising:
a recording head including a piezoelectric actuator for varying the volume of a pressure generating chamber by the deformation thereof to eject an ink drop from a nozzle communicating with the pressure generating chamber; and
a drive signal generator for supplying a drive signal supplied to the piezoelectric actuator to deform the same in accordance with the potential of the signal, the drive signal including:
a pull-in element for varying the potential of the drive signal such that the pressure generating chamber is expanded to pull in a meniscus of the ink in the nozzle thereto;
a first ejection element for varying the potential of the drive signal such that the expanded pressure generating chamber is partly contracted; and
a second ejection element for varying the potential of the drive signal such that the contracted pressure generating chamber is expanded again to eject the ink drop from the nozzle.
Preferably, the drive signal includes:
a pull-in hold element connecting an end potential of the pull-in element and a start potential of the first ejection element at the same potential; and
an ejection hold element connecting an end potential of the first ejection element and a start potential of the second ejection element at the same potential.
Preferably, the potential difference of the second ejection element is equal to the potential difference of the first ejection element or less.
Preferably, the potential of the second ejection element at an end potential thereof is the potential of the first ejection element at a start potential thereof or less, and is higher than the end potential of the first ejection element at an end potential thereof.
Preferably, a duration of the pull-in element is matched up to a natural period of the pressure generating chamber.
Preferably, the drive signal includes a contraction element after the second ejection element for varying the potential such that the pressure generating chamber expanded by the second ejection element is contracted such an extent that an ink drop is not ejected from the nozzle.
Preferably, the contraction element includes at least two contraction elements and at least one contraction hold element connecting the preceding contraction element and the following contraction element at the same potential.
Preferably, the potential gradient of the following contraction element is equal to or less than the potential gradient of the preceding contraction element.
Preferably, the drive signal includes a previous contraction element for varying the potential such that the pressure generating chamber is once contracted before the application of the pull-in element.
Preferably, the drive signal includes a damping element for varying the potential such that the pressure generating chamber contracted by the contraction element is restored to an original volume thereof in order to stabilize the motion of the meniscus.
Preferably, the drive signal generator includes:
an output voltage information storage for storing a potential value of the drive signal;
a variation information storage for storing a variation amount of the potential of the drive signal; and
a calculator for calculating a potential value based on at least the potential value stored in the output voltage information storage and the variation amount stored in the variation information storage.
The calculation result is loaded in the output voltage information storage every predetermined period while changing the variation amount stored in the variation amount storage to generate a drive signal programmably.
Preferably, the drive signal includes a second drive pulse for ejecting an ink drop heavier than the ink drop ejected by a first drive pulse defined by the pull-in element, the first ejection element and the second ejection element.
Preferably, the drive signal includes a third drive pulse for ejecting an ink drop heavier than the ink drop ejected by the application of the second drive pulse.
Preferably, the third drive pulse, the first drive pulse and the second drive pulse are arranged in order in the drive signal.
According to the present invention, there is also provided an ink jet recording apparatus comprising:
a recording head including a piezoelectric actuator for varying the volume of a pressure generating chamber by the deformation thereof to eject an ink drop from a nozzle communicating with the pressure generating chamber; and
a drive signal generator for supplying a drive signal supplied to the piezoelectric actuator to deform the same in accordance with the potential of the signal, the drive signal including:
a pull-in element for varying the potential of the drive signal such that the pressure generating chamber is expanded to pull in a meniscus of the ink in the nozzle thereto;
a first contraction element for varying the potential of the drive signal such that the expanded pressure generating chamber is partly contracted;
a hold element for holding the potential of the drive signal such that the contracted state of the pressure generating chamber is retained to eject the ink drop from the nozzle;
a second contraction element for varying the potential of the drive signal such that the pressure generating chamber is contracted again to stabilize vibration of the meniscus of the ink.
Preferably, a time period from a start end of the first contraction element to an end potential of the hold element is equal to a half of a natural period of the piezoelectric actuator.
Preferably, a time period from a start end of the first contraction element to an end potential of the second contraction element is equal to a natural period of the pressure generating chamber.
Preferably, the potential gradient of the first contraction element is steeper than the potential gradient of the second contraction element.
Preferably, the drive signal includes a second drive pulse for ejecting an ink drop heavier than the ink drop ejected by a first drive pulse defined by the pull-in element, the first contraction element, the hold element and the second contraction element.
Preferably, the drive signal includes a third drive pulse for ejecting an ink drop heavier than the ink drop ejected by the application of the second drive pulse.
Preferably, the first drive pulse and the second drive pulse are arranged in order in the drive signal.
Preferably, the drive signal includes a fourth drive pulse for vibrating the meniscus of the ink without ejecting the ink drop from the nozzle. The first and second drive pulses are preceded by the fourth drive pulse.
According to the present invention, there is also provided a method of driving a recording head including a nozzle communicating with a pressure generating chamber, from which an ink drop is ejected due to volume variation of the pressure generating chamber, comprising the steps of:
expanding the pressure generating chamber to pull in a meniscus of the ink in the nozzle thereto;
contracting partly the expanded pressure generating chamber; and
expanding again the contracted pressure generating chamber to eject the ink drop from the nozzle.
According to the present invention, there is also provided a method of driving a recording head including a nozzle communicating with a pressure generating chamber, from which an ink drop is ejected due to volume variation of the pressure generating chamber, comprising the steps of:
expanding the pressure generating chamber to pull in a meniscus of the ink in the nozzle thereto;
contracting partly the expanded pressure generating chamber;
retaining the contracted state of the pressure generating chamber to eject the ink drop from the nozzle; and
contracting again the pressure generating chamber to stabilize vibration of the meniscus of the ink.